how many cells?

Hi-- I teach high school math in AMherst, Mass. In my
Quantitative Reasoning class, we're exploring the realm of big
numbers. I challenged my kids to come up with something to
count that there might be a googol of. I don't think it will
be anything physical (since even atomic particles is only
10^87 or so), but they suggested counting all the cells in all
the organisms in the world. I know it's well short of a googol,
but I have no idea of the order of magnitude.
Can anybody give me any facts I can use to put together a
rough estimate? Like how many organisms are there in the
world? how many cells in various broad categories of things?
we could start with humans, I guess.
tell me anything you know that might help here.
thanks!!
email to: nkoch at k12.oit.umass.edu
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